I've always thought about how Bronze Age had trouble supporting an army of culture X in a province populated by culture Y; cultural assimilation certainly makes sense.
I should add that Wikipedia has the list of historically largest cities; the recordholder was consistently in the tens of thousands (and was pretty consistently a capital, much like Rome was in the iron age), so later on you may wish to consider a simple multiplier for the capital region. Then again, you may have already planned for that! ;)